During the past half century, there has been
tremendous growth in exploring the chemistry, physics and material
science of poly(vinylidene fluoride) (PVDF)-based ferroelectric
polymers. With high breakdown strength, easy processing, and
outstanding insulating features, these all-organic electrically
active materials are attractive for many advanced electrical (e.g.,
pyroelectric and piezoelectric) applications. Concurrently, the
research about ferroelectricity in polymers also began for energy
storage applications through controlling crystalline structure and
tuning the corresponding ferroelectric properties, e.g., novel
relaxor ferroelectricity with slim double- or single-
hysteresis-loop (DHL and SHL) behavior in PVDF copolymers and
terpolymers. On the other hand, research activities also focused on
searching for other classes of ferroelectric polymers, such as
ferroelectric polyamides, cyanopolymers, polyureas, and
polyuerathane. However, not much progress has been achieved as
considerable as what has been achieved for PVDF-based polymers. In
this thesis, we focus on the novel ferroelectric behaviors in
polyamide-based polymers, trying to deeply understand the origin of
ferroelectricity in aliphatic and aromatic nylons, and generalize
the relaxor ferroelectric behavior into this hydrogen-bonding
system. It has been commonly considered that only odd-numbered
nylons, which prefer polar crystalline structure, are able to show
ferroelectric hysteresis loops. In contrast, even-numbered nylons,
such as nylon-12 and nylon-6 should not exhibit any
ferroelectricity due to their nonpolar crystalline structure.
However, in our study, ferroelectric properties are also reported
for mesomorphic even-numbered nylons. The structure of the
mesophases in quenched samples was considered to contain multiple
twists in the chain conformation and dangling/weak hydrogen bonds,
which enabled dipolar switching, forming electric-field-induced
ferroelectric domains. This study shows that different from
ferroelectric PVDF-based polymers, the polar crystalline structure
is not the prerequisite for ferroelectricity in nylons. Instead,
mesophases with enlarged interchain spacing and disordered hydrogen
bonds are the key to ferroelectricity. With the understanding of
ferroelectricity in n-nylons, further weakened hydrogen bonding and
more twisted chain conformation (e.g., the all-trans conformation
of nylon-12 under poling) are expected to facilitate dipolar
relaxation and prevent the large ferroelectric domain from growing
up. The simplest way to weaken hydrogen bonds is to raise the
temperature. Upon increasing temperature to 100 °C, the D-E loops
became increasingly narrower, finally leading to slim DHLs for
nylon-6 and nylon-12. The observed DHL behavior was attributed to
the electric-field-induced reversible transitions between the
paraelectric (less twisted chains) and ferroelectric (more twisted
chains) states in the mesomorphic crystal of even-numbered nylons.
Furthermore, the SHL relaxor ferroelectric property was
successfully achieved…
Advisors/Committee Members: Zhu, Lei (Advisor), Protasiewicz, John (Committee Chair).